Need help understanding Oscilloscope behavior!!

[sohamsurve]

Hello All,
I have the following issue while using a function generator connected to a oscilloscope.
I have used plain BNC connectors and cables for the entire purpose.
The following are the setting on my generator: 25Mhz; 1Vpp, 50Ohms.
Now, when I connect to Channel 1 on the oscilloscope I am getting decently acurate similar readings as mentioned above.
But for my purpose I need to connect a BNC T-connector so that I can measure the input and give an input to a circuit that I need to use at the same time.

So I connect this T-connector at the Channel 1 of the oscilloscope. On one side of the T-conn I connect the input signal from the generator, on the other side of the T, I connect another BNC which goes to my circuit.

Now ideally, all these T-connector terminals should show similar voltage, but in my case the voltage on the end of the cable that goes to the circuit is stepped up from the original voltage (ranges from about 1.5-3 volts for a 1V p-p input).
I found this by simply connecting the BNC which goes to circuit to another channel on the oscilloscope.

Kindly let me know what is happening.
PS: All my terminations are fine.

Thank you

 

[rho-bot]

I doubt the terminations are fine...
Usually this is caused due to reflections by wrong termination.

It should look like:
                                     Scope(1Meg)
                                        |
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit(50R terminated)

I guess it looks like:
                                     Scope(50R terminated)
                                        |
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit(Hi-Imp)

 

[sohamsurve]

The set up is as follows:

Scope Ch.1 (50R)
|
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit (Scope Channel 2 [1Meg])**

**So because I want to check the amplitude levels at the circuit end of the T-connector, I am connecting that end to Channel 2 of the Scope instead of the actual circuit, in order to simply observe if my circuit is being fed the correct input. But turns out that the level in Ch. 1 is going down by 0.1-0.5 V and Ch. 2 is stepping up.

 

[FvM]

Tapping the 50 ohm impedance matched connection with a BNC-T and a high impedance oscilloscope input is only a halfway correct termination scheme, but it should give acceptable signal quality for medium fast signals. Correct wideband matching would use a -6dB resistive power splitter and 50 ohm oscilloscope termination.

sohamsurve should report the actual setup.

- - - Updated - - -

As sketched by rho-bot, the end nodes need to be 50 ohm terminated and the device at the T-connector high impedance.

 

[D.A.(Tony)Stewart]

It means the unterminated coax and load reactance plus coax capacitance has a resonant effect with a gain up to 3. If you sweep the f , then you will find even higher voltages at the self resonant frequency.

This is why we don't use this method to drive high impedance loads on the end of a long cable without tuned cable lengths or careful consideration to the "impedance transformer" effects of long mismatched cable.

After the parallel resonant frequency, you may find a series shunt resonant frequency at the 1/4 wavelength with no signal using an RF sweep.

 

[biff44]

After the parallel resonant frequency, you may find a series shunt resonant frequency at the 1/4 wavelength with no signal using an RF sweep.

yes I too was wondering HOW LONG that cable was with the high impedance voltmeter trying to measure it? The line IS a transmission line, and will behave like a transmission line. With an OC load, there is a forward wave, and a equal reverse wave at the end. That can look like 2X the expected voltage

 

[D.A.(Tony)Stewart]

A matched transmission line will double when termination is disconnected but not 3x. There must be an inductive element raising the Q at the end with partial resonance with cable capacitance.

Since impedance rises when this occurs with parallel resonance, there is no power gain.